1. Field of the Invention
The present invention relates generally to two-wire signal transmitters for telemetry and, more particularly, to transmitters which provide direct current isolation between the transducer input circuitry and the transmitter output circuitry.
2. Description of the Prior Art
In the past, relatively small transducer voltages such as those from temperature sensing thermocouples, or the like, were amplified or processed at their source before being transmitted to the remote indicating location. In a particular type of telemetry system, the transmission line between the transmitter and the indicating location not only conducts the processed transducer signal but also conducts operating power to the transmitter itself. Such a system is conventionally known as a "two-wire" system to indicate that signals may be transmitted in both directions simultaneously.
In some cases, the transducer may be connected to a device which is maintained at a direct current voltage which is considerably different than that of the indicating location. In such cases, the signal transmitter must be direct current voltage isolated from the transmission line. An additional reason for maintaining direct current isolation is that spurious voltages such as those from lightning may occur on long transmission lines which would adversely affect the normally delicate circuitry of the signal transmitter.
A particular class of two-wire telemetry system to which the present invention is generally related which is basically conventional is that of temperature sensing in which a common direct current power supply at the indicating location provides the direct current operating voltage to one or a plurality of transmission lines to power the respective temperature signal transmitters. In such a system, the transducer signal typically controls the current in the transmission line which is sensed at the remote location to provide an indication of the signal. Illustrative systems of this type are shown in Kobayashi et al, U.S. Pat. No. 3,483,476, which illustrates the basic technique of isolating the input signal processing circuitry from the output circuitry connected to the transmission line and providing power to the input signal processing circuitry by means of a direct current to alternating current to direct current conversion technique using an isolating transformer in the alternating current circuitry. A similar technique is used in converting the signal from direct current in the input circuitry to direct current in the transmitter output circuitry.
An example of a more recent technique utilizing the same principles is shown in Chana, U.S. Pat. No. 3,959,786, which provides power to the input signal circuitry by means of an oscillator driving a transformer, the secondary of which includes rectifiers to generate the direct current voltage necessary to operate the input circuitry. The input signal itself is processed in the direct current mode till applied to the primary of a current transformer which is operated as a chopper by connecting it to the terminals of the secondary of the power transformer. The resultant alternating current signal on the secondary of the signal transformer is rectified and applied to a series current generator in the transmission line.
A further aspect of such a two-wire temperature telemetry transmitter is that, while a number of different transducers are available for temperature sensing, the temperature correspondent signals from these transducers are typically quite small, in the millivolt range for thermocouples, for example, and that temperature changes produce relatively small changes in those millivolt signals. Similarly, while a resistance temperature detector may have a substantial reference resistance, temperature changes typically produce relatively small changes in the resistance detector. Another aspect of temperature sensing is that the transmitter circuitry must operate over a relatively wide range of environmental conditions with attendant, component variations.
There has been a need in the field of telemetry signal transmitters for two-wire systems in which the transducer signal is relatively small for a transmitter system which provides precise and stable telemetry signals to the transmission line for a wide range of operating conditions including direct current isolation of the transducer from the transmission line. The present invention provides such a system.